Is it OK to email a QuantLib developer to ask questions, or seek help, or report a bug?
Well, it’s not a crime. However, we urge you to consider posting to the QuantLib mailing list instead. Messages on the list are stored, and thus benefit the whole project: the next user with your problem will be able to find the answer by searching the archives. Moreover, the developers can volunteer only a limited time; thus, they will employ such time to answer questions on the list (which serves the project) rather than doing private tutoring for free. They are likely to answer you “please, post your question on the mailing list” anyway. They might even refer you to this FAQ entry.